Problems with fogging have plagued the photographic industry from its inception. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. Fog can be defined as a developed density that is not associated with the action of the image-forming exposure, and is usually expressed as "Dmin", the density obtained in the unexposed portions of the emulsion. A density, as normally measured, includes both that produced by fog and that produced by exposure to light.
It is known in the art that the appearance of photographic fog related to reduction of silver ion can occur during many stages of preparation of the photographic element including silver halide emulsion preparation, (spectral) chemical sensitization of the silver halide emulsion, melting and holding of the liquid silver halide emulsion melts, subsequent coating of silver halide emulsions, and prolonged natural and artificial aging of coated silver halide emulsions. Particularly, silver halide emulsions precipitated in the presence of ripeners such as thioethers or ammonia and/or in environments sensitive to reduction of silver ions such as high pH and/or low pAg usually suffer from high fog and poor raw stock keeping (RSK).
Several methods have been employed to minimize this appearance of fog. Mercury containing compounds, such as those described in U.S. Pat. Nos. 2,728,663; 2,728,664; and 2,728,665, have been used as additives to combat fog. Thiosulfonate and thiosulfonate esters, such as those described in U.S. Pat. Nos. 2,440,206; 2,934,198; 3,047,393; and 4,960,689, have also been employed.
Aromatic, heterocyclic, and acyclic disulfides which do not have labile sulfur or sulfide, such as those described in U.S. Pat. Nos. 1,962,133; 2,465,149; 2,756,145; 3,043,696; 3,057,725; 3,062,654; 3,128,186; and 3,563,754, have been used primarily as emulsion melt additives, i.e., being introduced into already (spectral) chemically sensitized silver halide emulsions prior to coating. U.S. Pat. No. 3,397,986 discloses bis(p-acylamidophenyl)disulfides as useful antifoggants added before or after any optically sensitizing dyes. However, the use of optically sensitizing dyes during chemical sensitization was not readily known in the art until their widespread use during tabular shaped silver halide emulsion sensitization. U.S. Pat. No. 3,397,986 and the others cited previously did not anticipate the utility of these non-labile disulfides during the sensitization of silver halide emulsions, either with or without optically sensitizing dyes. The prior art use of these disulfides as melt additives does decrease fog and stabilize against fog during aging of coated emulsions, but when used in this manner also decreases sensitivity and requires the use of additional stabilizers like azaindenes, such as described in U.S. Pat. No. 3,859,100.
Another challenge in the manufacture of photographic emulsions is the control of the shape and size of the silver halide grains. Morphology (crystal shape) of silver halide emulsion grains plays an important role in their photographic applications. For example, high aspect ratio tabular grain silver haloiodide emulsions have been recognized to provide a variety of photographic advantages, such as improved speed-granularity relationships, increased image sharpness, and reduced blue speed of minus blue recording emulsion layers as illustrated in Kofron et al, U.S. Pat. No. 4,439,520. Research Disclosure 25330, published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a North Street, Emsworth, Hampshire P010 7DQ, ENGLAND, by Buhr et al, discloses how to utilize optical properties of tabular emulsions for optimizing photographic responses of specific layers depending on their grain thicknesses.
Silver chloride crystals are mostly of cubic morphology bound by (100) faces when precipitated under standard conditions. In some cases, other faces may appear. It is most common, however, that (111) and perhaps (110) crystallographic faces occupy only a small fraction of the total crystal surface. The relative amount of those faces may depend on the presence of selective growth modifiers, which would be able to slow down the growth of fast growing faces like (111) and (110). Such growth modifiers are known in the art; W. Reinders, "Study of Photohalides III, Absorption of Dyes, Proteins, and Other Organic Compounds in Crystalline Silver Chloride", Zeitschrift fur Physikalische Chemie, volume 77, pages 677-699 (1911), J. Maskaski CA 1,280,312 and references therein. It is also known that different crystallographic faces respond differently to chemical sensitization. Therefore, it is desirable to be able to control some aspects of chemical sensitization with crystal morphology.
T. H. James, "The Theory of the Photographic Process", pages 98-100, Fourth Edition, Macmillan Publishing Company, Inc., New York (1977), describes how various crystal morphologies are formed in different grain formation conditions such as in an excess of bromide ions and solvents such as ammonia. As an example, AgBrI emulsions can be made in various morphologies depending on pAg (silver ion activity). "Particle growth in Suspensions", page 159, Academic Press, London, (1973), discusses growth modifying agents including cationic surfactants and thioureas as growth accelerators and 1-phenyl-5-mercaptotetrazole (PMT), 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene (TAI) and certain cyanine dyes as growth restrainers. U.S. Pat. No. 4,749,646 (Herz et al) discloses the use of a 1,1,3,3-tetrasubstituted middle chalcogen urea compound as an effective grain growth modifying agent.
U.S. Pat. No. 4,912,017 (Y. Takagi and S. Nishiyama) discloses the use of sulfur-containing compounds such as PMT and mercaptothiadiazoles and thio-ketone group containing compounds to prevent grain size fluctuation (development unevenness) after formation of grains. It also discloses the use of disulfide compounds which are easily cleavable into the above compounds. These compounds can allegedly adsorb to the surface of silver halide crystals. This patent teaches that the preferred point of addition is during the preparation of a diluted coating emulsion.
There is a continuing need for methods of improving the fog characteristics of photographic emulsions. Further, there is a continuing need for additional methods of controlling grain formation to improve photoefficiency. In accordance with this invention, it has been found that the addition of certain organic dichalcogenides during precipitation of a silver halide emulsion can modify the grain growth of silver halide crystals. This allows the independent control of surface morphology, thickness, size, and dispersity.
It has also been found that the addition of these same dichalcogenide compounds to a silver halide emulsion during precipitation gives lower fog without a concomitant large loss in sensitivity. It has further been found that equivalent fog reduction can be obtained with less dichalcogenide when the dichalcogenide is used during precipitation, rather than as a melt additive, and that less or no latent image destabilization occurs. Additionally, less loss in sensitivity occurs after aging of the coated emulsions.